Doctor Doom’s face has been hidden behind an iron mask ever since an experiment literally blew up in his face when Victor von Doom created a machine to communicate with his dead mother. He ignored warnings by Reed Richards AKA Mister Fantastic that his calculations were off due to tampering by Ben Grimm AKA The Thing, unbeknownst to them.
The explosion left only a slight scar on his cheek according to an interview with creator Jack Kirby. Being a perfectionist, Doom sealed away his face behind the still cooling iron mask further mutilating himself as depicted in Fantastic Four Annual #2. The actual scar wasn’t illustrated until later in a retelling of Doctor Doom’s origin by John Byrne in Fantastic Four #278.
Since his disfigurement, Doctor Doom has seldom shown his face except when temporarily healed trough various means. Doom restored himself after stealing the power of a cosmic entity known as the Beyonder in the orignal Secret Wars. He also healed himself by transferring his mind into new bodies but this was put to a stop by another cosmic entity known as the Living Tribunal. He punished a cosmically empowered Doom for injuring the cosmic historian, Uatu the Watcher, in FF #16. The Living Tribunal explained that with godlike power there must come godlike responsibility.
“Let the punishment fit the crime,” he declared. “From this day forth, each base act of malice on your part will carve a new scar upon your heretofore unblemished visage.”
The Tribunal concluded that no other power could heal or harm his face.
Doom once again acquires godlike power in the new Secret Wars series. This time he uses it to save the universe from the Beyonders. He combines the entire Marvel Universe past, present and future into a single planet known as Battleworld where he rules supreme as a god. The only people are aware of the previous universe are his right-hand man Doctor Strange, who helped him create Battleworld, and Doom’s wife Susan Richards AKA the Invisible Woman, who knows only what Doom tells her.
Doom confides in Susan that he is the only thing flawed in his perfect world. Despite being omnipotent, he cannot fix his face. Doom removes his mask to reveal his face in Secret Wars #3. Artist Esad Ribic draws a pitiful looking man rather than the monstrous villain we expect. The disfigurement could be due to the punishment handed down by the Living Tribunal; however, Doom’s godlike power would the presumably negate it especially because the Beyonders have killed the Living Tribunal. It’s possible that Doctor Doom is subconsciously manifesting his true face.